spanconfig: add a static split point at the start of meta2

[arulajmani]

Meta1 is not allowed to split, whereas meta2 is. This could prevent load based splitting from applying to meta2. This patch now installs a static split point at the start of meta2 so that load based splitting can do its thing.

Resolves https://github.com/cockroachdb/cockroach/issues/119421

Release note: None

[cockroach-teamcity]

This change is 

[arulajmani]

I'm closing this for now. We realized late in the release cycle that there's a bit of complexity here during cluster bootstrapping -- the second (unpolished) commit is a stab at it, though we'd want to understand those changes much better before merging.